I first started teaching computer graphics in the early 90's, when I was barely older than my students. I knew the material so well I didn't think that planning for my very first class would be an issue. Turns out, my "material" for the first class was totally used up 1/2 hour into a 3 hour session, and there was about 10 seconds of silence as I tried to figure out what we should be doing. Longest 10 seconds of my life, and never again was I unprepared for a first (or any) class.
